It is general that chrome-containing steel such as typically stainless steel is produced in a process including melting scrap and other materials in an electric furnace into a molten iron (in other words, it may be called to a molten pig iron) followed by refining it into a steel having a predetermined component composition. In producing a chrome-containing molten iron in an electric furnace, Cr2O3-containing slag is formed. In actual operation, in order to reduce the cost of chrome-containing steel, it is important to collect chrome from the electric furnace slag into the molten iron as much as possible to thereby increase the chrome yield rate in the steel.
Heretofore, in producing chrome-containing molten iron in an electric furnace, there has been employed a method of using CaF2 as a flux component so as to increase the flowability of slag. Recently, however, in use of steel slag as a ground or roadbed material, the content of the fluorine ingredient therein has become restricted, and therefore utilization of CaF2-free slag, or that is, slag not containing CaF2 has increased in electric furnace operation. In that case, the melting point of slag rises and the flowability thereof lowers. In addition, the basicity of slag, CaO/SiO2 lowers and the SiO2 activity in slag tends to increase. For these reasons, as compared with CaF2-incorporated slag, CaF2-free slag has a problem in that chrome-reducing reaction according to the following formula (1) (the rightward reaction) is difficult to go on therein and therefore the chrome yield rate in molten iron tends to lower.3[Si]+2(Cr2O3)=3(SiO2)+4[Cr]  (1)
For promoting chrome reduction in slag, a method may be taken into consideration of adding a metal Al-containing substance to thereby promote the reduction reaction according to the following formula (2):2Al+(Cr2O3)=(Al2O3)+2[Cr]  (2)
However, the melting point of CaF2-free slag is high and the flowability thereof is low, and therefore in the slag of the type, the reaction of the formula (2) is difficult to promote sufficiently in the electric furnace process, and at present, the method could not be so much an effective countermeasure.
On the other hand, another method is known, which includes transferring chrome-containing molten iron produced in an electric furnace into another refining vessel along with slag thereinto, stirring it with inert gas blowing thereinto or with stirring with an impeller for chrome reduction and collection from slag (Patent References 1, 2). As a reducing agent, used is an Al or Si-containing substance, and the Cr2O3 content in the slag after stirring treatment is reduced to around 2.8 to 4.4% (Patent Reference 1, Nos. 1, 2, 4 and 5 in Table 1), or around 2.6 to 4.7% (Patent Reference 2, Examples in Table 3).